The present invention is directed to a printing plate precursor and to a process for preparing a printing plate. More specifically, the present invention is directed to a printing plate precursor comprising a base layer, a layer of photohardenable material, and a layer of softenable material containing photosensitive migration marking particles. In one embodiment of the present invention, a printing plate is prepared by electrically charging the precursor and then exposing the precursor to light in an imagewise pattern. After exposure, the softenable material is made to soften, thereby enabling the migration marking particles that had been exposed to light to migrate through the softenable material toward the base layer and resulting in the layer of softenable material becoming transmissive to light in areas where the migration marking particles have migrated toward the base layer. Subsequently, the precursor is uniformly exposed to light, thereby causing areas of the photohardenable material to harden in areas situated contiguous with light-transmissive areas of the softenable layer. Thereafter, the precursor is exposed to a solvent in which the softenable material and photohardenable material in its unhardened form are either soluble or are softened sufficiently to enable their removal from the base layer by wiping or brushing, and in which photohardenable material in its hardened form is not soluble, thereby removing from the base layer all materials except for the hardened photohardenable material, which remains on the base layer in imagewise pattern. Alternatively, if desired, the plate can subsequently be exposed to an etchant that etches the base material in areas not covered by the photohardenable material, followed by removal of the hardened photohardenable material from the base layer, leaving the base layer etched in an imagewise pattern. This etching process is often used for processing lithographic printing plates of the deep-etch or bimetallic type, as disclosed in, for example, The Lithographer's Manual, 7th Ed., R. N. Blair, ed., Graphic Arts Technical Foundation, Pittsburgh (1983), the disclosure of which is totally incorporated herein by reference.
In conventional lithographic printing processes, printing plates are frequently prepared by first forming on conventional silver halide film an image corresponding in size to the desired size of the images to be generated, generally by photographing a paste-up of the desired image. Subsequent to development of the silver halide film, the film is transmissive to light in some areas and absorbing to light in other areas in an imagewise pattern. A printing plate precursor, which typically comprises a base layer and a layer of photohardenable material, such as a diazo compound or diazo sensitizer in an organic colloid or synthesized polymer, or a polymer that becomes crosslinked upon exposure to light, is then placed in contact with the developed silver halide film, and light, generally within the ultraviolet wavelength range, is directed onto the silver halide film. The light passes through the silver halide film mask to the photohardenable material in areas of the film that are transmissive to light, and the photohardenable material exposed to light becomes hardened while unexposed areas of the photohardenable material remain unhardened. Subsequently, the precursor is exposed to a solvent in which the hardened form of the photohardenable material is insoluble and the unhardened form of the photohardenable material is soluble, thus washing away the unhardened material and leaving the hardened material on the base layer in a pattern corresponding to the desired image. The hardened photohardenable material is typically hydrophobic, while the base layer is generally hydrophilic, although the base layer can be selected to be hydrophobic and the hardened photopolymeric material can be selected to be hydrophilic. Thus, when the printing plate thus formed is contacted with an oil-based ink, the ink remains on portions of the plate containing the hardened photohardenable material but is repelled by the base plate material. Contacting the plate with an ink and then contacting the inked plate with a printing substrate thus generates prints of the desired image. Alternatively, the ink image on the plate can be applied to an offset roller and the ink on the offset roller subsequently applied to the printing substrate. Further, instead of using a photohardenable material on the base plate, a hydrophobic photodegradable material can be used in which the exposed areas can be removed on development. Plate coatings of the type described are generally negative working in that the light exposed areas become photohardened and ink receptive and form the image areas. The plate coatings, however, can also be positive working. In this instance, the exposed areas are photodegraded and washed away on development and become the hydrophilic or non-image areas of the plate. The unexposed areas remain after development and require fixing to render them light insensitive. These areas generally are hydrophobic and ink receptive and hence form the image areas.
These known processes have the disadvantage that generation of the desired image on silver halide film prior to exposing the printing plate results in added expense and processing times for printing processes wherein formation of a silver halide image is not otherwise necessary or desirable, such as digital pagination systems wherein the image is computer generated. Accordingly, a printing plate precursor that can be exposed directly by, for example, a scanning laser driven by a digital page file, would exhibit advantages such as convenience, rapid processing time, and lower cost. While it may be possible to expose a conventional printing plate by such a process, the exposure generally would require very high power lasers, which tend to be expensive and short-lived. Further, while it may be possible to employ conventional argon ion or helium-cadmium lasers to expose a printing plate comprising a series of photographic type silver halide emulsions on a paper base, these plates are often short-lived during the printing process. One type of direct imaging plate is described in The Lithographer's Manual, 7th Ed., R. N. Blair, ed., page 10:28, Graphic Arts Technical Foundation, Pittsburgh (1983). Because there is only a small difference between the ink and water receptivity of the image and nonimage areas on this type of plate, it is difficult to achieve optimal conditions with respect to exposure, processing, and printing on a press. With considerable care, acceptable results can be obtained as long as the contrast range of the copy is not too great; it is difficult to mix line, halftone, and solid areas on one plate, as each requires different levels of exposure or different inks for optimum printing results. Thus, a printing plate having the printing characteristics of a conventional printing plate but capable of camera speed exposure for the initial exposure is particularly desirable and is provided by the present invention.
U.S. Pat. No. 4,532,197 (Humberstone et al.) discloses a method of forming an image on an electrophotographic film material. The process entails a contact printing technique and comprises placing an image-bearing master in contact with the film, exposing the film to light through the image-bearing master, the exposure being substantially greater than the minimum necessary to render conductive the photoconductive layer of the electrophotographic film, applying a substantially uniform charge to the surface of the film in the dark immediately after exposure, leaving the film in the dark for a short time so as to allow the charge to migrate selectively, and then developing the image.
Further, U.S. Pat. No. 4,230,782 (Goffe), the disclosure of which is totally incorporated herein by reference, discloses a migration imaging system wherein an imaging member comprising migration marking material contained in or contacting a softenable layer on a supporting substrate has a latent image formed thereon, and the imaging member is subsequently developed by passing it through one or more small meniscuses bonding at least in part a volume of liquid which is capable of changing the resistance of the softenable material, to enable the migration marking material to migrate toward the substrate. Alternately, an imaged migration imaging member having marking material in a migrated image configuration and in a background configuration, which is at least in part spaced apart in depth in the softenable layer from the image configuration, is further developed by this system to enhance image quality.
In addition, U.S. Pat. No. 4,762,764 (Ng et al.) discloses a liquid developer suitable for developing electrostatic latent images either on dielectric paper or on an electroreceptor or photoreceptor substrate. In Examples 1, 3, and 6 to 10 of the patent, the liquid developer is used to develop images on a migration imaging member.
"Applications of Xerox Dry Microfilm (XDM), a Camera-Speed, High Resolution, Nonsilver Film with Instant, Dry Development," A. L. Pundsack, P. S. Vincett, P. H. Soden, M. C. Tam, G. J. Kovacs, and D. S. Ng, Journal of Imaging Technology, vol. 10, no. 5, pages 190 to 196 (October 1984), the disclosure of which is totally incorporated herein by reference, discloses migration imaging members and the imaging steps associated therewith. This article also discloses the use of a migration imaging member instead of silver halide film as a film intermediate in the formation of printing plates. In addition, this article proposes a printing plate comprising a substrate and a migration imaging member, wherein an electrostatic toning process is employed to create the required ink-attracting properties in the image areas and ink repelling properties in the nonimage areas. Since the softenable matrix polymer is generally hydrophobic, the toner should be hydrophilic. The toner can be fused to the matrix polymer surface to form the printing plate. In contrast to the printing processes described in this article, the present invention entails exposing to light a conventional printing plate with an in-situ mask comprising a migration imaging layer, which layer is subsequently washed away prior to employing the exposed printing plate in printing processes, resulting in formation of a conventional printing plate.
U.S. Pat. No. 3,820,984 (Gundlach) and U.S. Pat. No. 3,648,607 (Gundlach), the disclosures of each of which are totally incorporated herein by reference, discloses a migration imaging system having a migration imaging member with a binder layer of softenable material wherein a mixture of electrically photosensitive and inert fusible particles is dispersed and an imaging process wherein the fusible particles are fused, thereby fixing the migrated image of the two types of particles. The imaged member is used as a lithographic printing master.
U.S. Pat. No. 4,518,668 (Nakayama), the disclosure of which is totally incorporated herein by reference, discloses a method for preparing a lithographic printing plate by providing a light-sensitive material comprising an electroconductive support having a hydrophilic surface and a light sensitive layer and a photoconductive insulating layer thereon. The material is imagewise exposed and then subjected to electrophotographic processing to form an electrostatic latent image on the photoconductive insulating layer. After exposure, the electrostatic latent image is developed with developer particles which are opaque to the light to which the lightsensitive layer is sensitive in the presence of an electrode facing the photoconductive insulating layer. The development is carried out while applying a bias voltage between the electrode and the light-sensitive layer so that the residual charge on the non-latent areas appears zero. The exposed or unexposed areas of the light sensitive layer are then removed together with the photoconductive insulating layer, resulting in a lithographic printing plate.
The expression "softenable" as used herein is intended to mean any material which can be rendered more permeable, thereby enabling particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, swelling, melting, or softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents, and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
The expression "fracturable" layer or material as used herein means any layer or material which is capable of breaking up during development, thereby permitting portions of the layer to migrate toward the base layer or to be otherwise removed. The fracturable layer is preferably particulate in the various embodiments of the printing plate precursors. Such fracturable layers of marking material are typically contiguous to the surface of the softenable layer spaced apart from the base layer, and such fracturable layers can be substantially or wholly embedded in the softenable layer in various embodiments of the printing plate precursors.
The expression "contiguous" as used herein is intended to mean in actual contact, touching, also, near, though not in contact, and adjoining, and is intended to describe generically the relationship of the fracturable layer of marking material in the softenable layer with the surface of the softenable layer spaced apart from the base layer.
The expression "optically sign-retained" as used herein is intended to mean that the dark (higher optical density) and light (lower optical density) areas of the visible image formed on the softenable layer of the printing plate precursor correspond to the dark and light areas of the illuminating electromagnetic radiation pattern.
The expression "optically sign-reversed" as used herein is intended to mean that the dark areas of the image formed on the migration imaging member correspond to the light areas of the illuminating electromagnetic radiation pattern and the light areas of the image formed on the migration imaging member correspond to the dark areas of the illuminating electromagnetic radiation pattern.
The expression "optical contrast density" as used herein is intended to mean the difference between maximum optical density (D.sub.max) and minimum optical density (D.sub.min) of an image. Optical density is measured for the purpose of this invention by diffuse densitometers with a blue Wratten No. 94 filter. The expression "optical density" as used herein is intended to mean "transmission optical density" and is represented by the formula: EQU D=log.sub.10 [.vertline..sub.o /.vertline.]
where .vertline. is the transmitted light intensity and .vertline..sub.o is the incident light intensity.
While known printing processes are suitable for their intended purposes, a need continues to exist for printing plate precursors and printing processes wherein the plate can be formed without the need for first forming an intermediate on silver halide film. In addition, there is a need for printing plate precursors and printing processes wherein the printing plate can be exposed directly by, for example, a scanning laser driven by a digital page file. Further, a need remains for printing plate precursors and printing processes that exhibit convenience, rapid processing times, and lowered cost compared to conventional printing processes employing silver halide film intermediates. There is also a need for printing plate precursors and printing processes wherein the printing plate can be exposed by a conventional laser apparatus wherein the photohardenable layer of the plate is of a conventional material and/or has the same printing characteristics of a conventional plate, such as plate life. A need also exists for printing plate precursors and printing processes wherein the imaging member and the printing plate coexist, thereby improving registration in the formation of multicolor images. When film intermediates are used to generate the image, registration is more difficult since there is an additional step where registration accuracy can be lost; in the instance of the present invention, however, there is no need to register intermediate film intermediates manually.